Uneasy lies the head that wears a crown. On Wednesday night, Joe Biden basked in the pageantry of a state dinner – white-jacketed violinists, golden chandeliers dotted with pink roses, a vivid wall display of 3D paper flowers. But soon after toasting the Australian prime minister in a pavilion on the White House south lawn, the US president had to step away to be briefed on a deadly mass shooting in Maine. The presence of Lloyd Austin, the defense secretary, and Jake Sullivan, the national security adviser, was a reminder of another, even darker shadow. Even as Biden and guests...